Method of fabricating a heat exchanger

ABSTRACT

Heat exchange inefficiencies found in round tube plate fin heat exchangers are eliminated in an aluminum heat exchanger that includes first and second headers ( 20 ), ( 22 ) and at least one flattened tube ( 24 ), ( 70 ) extending between the headers ( 20 ), ( 22 ). A plurality of generally parallel tube runs are defined and each has opposite edges. A plurality of plate fins ( 26 ), ( 50 ) are arranged in a stack and each has a plurality of open ended slots ( 34 ), one for each run of the tubes ( 24 ), ( 70 ). Each of the tube runs ( 24 ), ( 70 ) is nested within corresponding slots ( 26 ) and the fins ( 26 ), ( 50 ) with one of the edges ( 40 ) of the tube runs extending outwardly of the corresponding fin ( 26 ). The assembly is brazed together.

This a divisional application of U.S. Ser. No. 09/778,310, filed Feb. 7,2001, entitled “Heat Exchanger,” now U.S. Pat. No. 6,964,296.

FIELD OF THE INVENTION

This invention relates to heat exchangers, and more specifically, to abrazed aluminum plate fin heat exchanger.

BACKGROUND OF THE INVENTION

In heat exchangers that have a high aspect ratio (the ratio of width toheight), it is frequently necessary to locate the tube runs in agenerally horizontal plane to minimize cost. Typical of such heatexchangers are evaporators and condensers as may be found in the airconditioning systems of off-highway vehicles, air conditioning systemsfor recreational vehicles, and in truck refrigeration systems.Particularly when used as evaporators, conventional serpentine louveredfins coupled with horizontal tube placement provides problems with waterdrainage due to the hold up of water between the fins. That is to say,it is necessary in evaporator applications that provision be made todrain moisture condensing on heat exchanger parts to prevent freeze-upwhich would block air flow and drastically impede efficiency.

As a consequence of these and other considerations, high aspect ratioevaporators and other heat exchangers have historically been producedusing round tube plate fin technology. The tubes are orientedhorizontally and the plate fins vertically to allow water to drain downthe fins, around the tubes and out through the bottom of the heatexchanger. However, when compared to brazed, parallel flow type heatexchangers, performance of round tube plate fin heat exchangers suffersin three main areas.

For one, the round tubes substantially occlude the frontal area of theheat exchanger through which air passes. As a consequence, poor air sideheat transfer results.

A second problem is that mechanical bonds between the tubes and theplate fins conventionally employed in such heat exchangers are incapableof reliably providing intimate, good heat exchange contact between thetubes and the fins and as a result, poor fin-tube bonds frequentlyreduce heat transfer.

A third problem is that the use of round tubes requires relatively largefin heights (fin height being the length of the fin between the centerlines of two adjacent tubes). These large effective fin heights resultin poor fin efficiency.

Still another problem that has sometime occurred in heat exchangersgenerally is undesirably low air side area. The lack of sufficient areaimpedes heat transfer on the air side as a quick review of Fourier's lawwill readily show. Consequently, it would be desirable to increase airside surface area without increasing fin height to the point where poorfin efficiency results.

The present invention is directed to overcoming one or more of the aboveproblems.

SUMMARY OF THE INVENTION

It is one principal object of the invention to provide a new andimproved method of making a heat exchanger that eliminates fixturingrequirements during a brazing process.

It is another principal object of the invention to provide a new andimproved heat exchanger employing plate fins that are verticallyarranged in a heat exchanger having horizontal tube runs, andspecifically, such a heat exchanger where excellent bonding is providedbetween the tubes and the plate fins to avoid poor heat transfer at theinterface between the tubes and the fins.

It is still a third principal object of the invention to provide a newand improved heat exchanger employing plate fins that maximizes air sidearea without undesirably increasing fin height so as to improve theefficiency of heat transfer on the air side of the heat exchanger.

An exemplary embodiment of the invention that achieves the firstprincipal object mentioned above includes a method of fabricating a heatexchanger with the steps of:

a) providing a plurality of generally parallel tube runs of a flattenedheat exchange tube having a major dimension and a minor dimension;

b) providing a plurality of plate fins, each having a plurality of tubeslots approximately equal to the number of tube runs, each slot openingto an edge of the associated fin and having i) a shape corresponding tothe cross-section shape of a tube run to be received in the slot, ii) adepth less than the major dimension of the tube run to be received inthe tube slot, and iii) a width approximately equal to or slightly lessthan the minor dimension of the tube run to be received in the slot. Themethod further includes the steps of

c) fitting the tube runs snugly into corresponding slots in each of thefins such that an edge of each tube run extends a distance beyond theends of the slots in which it is received;

d) locating the assembly resulting from step c) on a supporting surfacewith the tube run edges in contact with the supporting surface and withthe plate fins extending above the tube runs; and

e) subjecting the assembly to an elevated temperature sufficient tobraze the fins to the tube runs while the assembly is on the supportingsurface and in the absence of brazing fixtures holding the fins on thetube runs in assembled relation.

In one embodiment, the tube runs are defined by straight sections of aserpentine tube while in another embodiment of the invention, the tuberuns are each defined by straight pieces of tubing.

In one embodiment of the invention, the cross-section of the tube runsis a tear-drop shape while in another embodiment, the cross section ofthe tube runs is oval shaped.

According to the second principal object identified above, there isprovided an aluminum heat exchanger which includes first and secondheaders and at least one flattened tube extending between and in fluidcommunication with the headers and defining a plurality of generallyparallel tube runs in spaced relation to one another. Each of the tuberuns has opposite edges defining a tube major dimension andinterconnecting side walls defining a tube minor dimension and aplurality of interior ports. A plurality of plate fins are arranged in astack and each has a plurality of open ended, tube run receiving slots,one for each tube run. Each slot has a shape generally that of thecross-section of the tube run to be received therein, a width equal orjust less than the minor dimension of the corresponding tube run and adepth somewhat less than the major dimension of the corresponding tuberun. Each of the tube runs is nested within corresponding slots in thefins with one of the edges of each tube run located outwardly of thecorresponding fin. The headers, the tube runs and the fins make up abrazed assembly.

In one embodiment, the plate fins are elongated and the slots open toone elongated edge thereof. The other elongated edge of the plate finsare uninterrupted by the slots.

In one embodiment, a stiffening bead is located between the otherelongated edge and the slots.

In still another embodiment, the plate fins are elongated and the slotsopen to both elongated edges of the fins.

In one such embodiment, the slots opening to one of the edges arealigned with slots opening to the other of the edges.

In one embodiment, the tube runs are defined by the legs of U-shapedtubes with one of the legs of each U-shaped tube being disposed in aslot opening to one elongated edge of the plate fin and the other legbeing disposed in a slot opening to the other elongated edge of theplate fin.

In such an embodiment, it is preferred that each of the legs of each ofthe U-shaped tubes includes a 90° twist immediately adjacent the bightof the corresponding U-shaped tube.

According to the third of the objects identified above, there isprovided a heat exchanger core that includes a plurality of generallyparallel tube runs formed of flattened, multi-port tubing and aplurality of plate fins in stacked relation having spaced openingssufficient to receive the tube runs. The tube runs are disposed in theopenings and have a major dimension brazed to the plate fins about theopenings and the parts of the plate fins between the openings arearcuate in a direction generally transverse to the major dimension tothereby increase the surface area of the fins between the openingswithout the need to increase the spacing between adjacent openings.

In one embodiment, the openings in the plate fins are slots extending tothe fins from one edge thereof.

Other objects and advantages will become apparent from the followingspecification taken in connection with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation of one embodiment of a heat exchanger madeaccording to the invention;

FIG. 2 is a sectional view of the embodiment of FIG. 1 takenapproximately along the line 2—2 of FIG. 1;

FIG. 3 is a sectional view of the embodiment of FIG. 1 takenapproximately along the line 3—3 in FIG. 1;

FIG. 4 is a view similar to FIG. 2 but of a modified embodiment of theinvention;

FIG. 5 is a view similar to FIG. 3 but of the embodiment illustrated inFIG. 4;

FIG. 6 is a side elevation of still another modified embodiment of theinvention;

FIG. 7 is a sectional view taken approximately along the line 7—7 inFIG. 6;

FIG. 8 is a sectional view taken approximately along the line 8—8 inFIG. 6;

FIG. 9 is a somewhat schematic view of still another modified embodimentof the invention;

FIG. 10 is a fragmentary, sectional view of a highly preferred plate finconstruction employed in any embodiment of the invention;

FIG. 11 is a view similar to FIG. 10 but of an optional, and somewhatless preferred, embodiment of the plate fin;

FIG. 12 is a fragmentary, sectional view of the cross-section of anembodiment of the invention employing a tear-drop shaped tube;

FIG. 13 is a sectional view taken approximately along the line 13—13 inFIG. 12; and

FIG. 14 is a graph showing the relative performance of four differenttypes of fins, namely, a conventional serpentine fin construction, theplate fin of FIG. 10, the plate fin of FIG. 11 and a conventional flatplate fin construction, all utilizing flattened, multi-port tubes.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Exemplary embodiments of the invention will be described in connectionwith the drawings, frequently in the context of heat exchangers havinghorizontal tube runs and vertically extending plate fins. However, it isto be understood that no restriction to such orientation is intendedexcept insofar as expressed in the claims. Similarly, while it ispreferable that the components of the heat exchanger be of aluminum oraluminum alloy, various performance enhancing features of the invention,such as the use of arcuate plate fins, and/or the use of plate finswhich are slotted and open to one side of the fin may be employed withefficacy in non-aluminum heat exchangers; and again, no restriction toaluminum heat exchangers is intended except insofar as expressed in theappended claims.

A first embodiment of a heat exchanger made according to the inventionis illustrated in FIGS. 1–3 and is seen to include a pair of verticallyextending headers, 20, 22 that are parallel and spaced from one another.The headers 20, 22 preferably are hollow cylinders formed and weldedfrom sheet aluminum or simply extruded, but could be multiple pieceheaders formed by welding or brazing if desired.

Flattened, multi-port tubes 24 formed as straight sections of individualpieces of tubing extend between and are in fluid communication with theheaders 20, 22. The tubes 24 may be formed by extrusion or may be weldedtubes provided with inserts.

Between the headers 20, 22 and fitted to the tubes 24 are a series ofaluminum plate fins 26. In a typical embodiment, the density of the fins26 will be about twenty fins per inch, although greater or lesser findensities can be employed as desired.

Preferably, between each of the tube runs 24, the fins 26 contain aconventional pattern of louvers 28 as best shown in FIG. 3.

FIG. 3 also illustrates the tubes 24 as having multiple, internal ports30. Typically, the hydraulic diameter of each of the ports will be nomore than about 0.070″ and even more preferably, will be 0.050″ or less.However, higher hydraulic diameters can be used if efficiency is not ofprime concern. The specific flattened tubes illustrated in FIG. 3 are inthe form of flattened ovals having flat external side walls 32, thespacing between which is referred to conventionally as the tube minordimension. This is illustrated as “d” in FIG. 3. The distance betweenthe curved ends or edges of each of the tubes 24 is conventionallyreferred to as the tube major dimension, shown as “D” in FIG. 3.

The fins 26 are arranged in a stack as seen in FIGS. 1 and 2 and eachfin in the stack has a series of slots 34 that open to one edge 36 ofthe fin 26. The opposite edge 38 of the fin 26, in the embodimentillustrated in FIGS. 1–3, is uninterrupted.

The slots 34 have a depth that is less than the tube major dimension,typically by an amount equal to about the radius of curvature of therounded edges 40 of the tubes 24. The slots 34 otherwise have a shapecorresponding to the cross-section of each of the tubes 24 but nominallyever so slightly smaller so as to assure that the edges of the slots 34tightly embrace the side walls 32 of the tubes 24. That is to say, thewidth of the slots 34 is preferably ever so slightly less than the tubeminor dimension “d”.

When the tubes 24 are formed of aluminum, the headers 20, 22 and fins 26will also be formed of aluminum. Preferably, the headers 20, 22 and finshave an external cladding of braze alloy and the tubes 24 are extrudedaluminum. Alternatively, the tubes 24 may be welded and have an externalaluminum braze alloy cladding thereon so as to form tight, brazed jointswith the headers 20, 22 and a good bond with the fins 26.

In assembling the heat exchanger illustrated in FIG. 1, the tubes 24 areinserted into aligned slots (not shown) in the headers 20, 22 and thestack of plate fins 26 applied thereto. Alternatively, the fins 26 maybe applied to the tubes before application of the headers 20, 22. In anyevent, because of the relative dimensioning of the tubes 24 and theslots 34 as mentioned previously, the tube edges 40 will extend past theedges 36 of the fins 26. As a consequence of this, the core thus formedmay be placed on a flat surface with the edges 40 of the tubes 24 incontact therewith for support. The same may be placed in a brazing oven(continuous or otherwise) and the temperature elevated to a brazingtemperature. Because, in a typical construction, the fins 26 will bethinner than the walls of the tubes 24, as the fins 26 approach themelting temperature of the base metal and begin to soften, they willsettle into the position illustrated in FIG. 3 through the action ofgravity and without the need for any special fixturing to cause thisresult. Brazing will occur and upon cooling, the assembly will appear asin FIG. 3 with all of the fins 26 in the stack aligned with one another.The process not only avoids misalignment of the fins in the finishedproduct which is unsightly, and thus undesirable, it eliminates the needfor fixtures during the brazing process to hold the fins in placerelative to the tubes, thereby considerably simplifying themanufacturing process.

The embodiment illustrated in FIGS. 1–3, inclusive, illustrates a singletube row heat exchanger. FIGS. 4 and 5 show an embodiment that providestwo tube rows in the heat exchanger. In the interest of brevity,identical components will not be redescribed and will be given the samereference numerals. In the embodiment illustrated in FIGS. 4 and 5, twoeach of the headers 20, 22 are employed, one for each tube row. Two rowsof the tubes 24 are employed as well and a stack of plate fins 50 areutilized. In the embodiment illustrated in FIGS. 4 and 5, the slots 34are formed in two rows, one opening to one edge 52 of the fin and theother row opening to the opposite edge 54 of the fins 50. The slots 34are dimensioned with respect to the tubes 24 in the same mannermentioned previously and again are provided with louvers 28 betweenadjacent ones of the tubes 24. Fabrication is as mentioned previouslyand by suitable plumbing, the rows maybe arranged in hydraulic parallel,in series, or may even be utilized to provide cooling for two differentfluids if desired.

In some instances, two adjacent headers, such as the headers 20, may bereplaced with a single larger header that receives the tubes 24 of bothrows. In such a case, one of the headers 22 would be provided with aninlet while the other header 22 would be provided with an outlet.

Still another embodiment of the invention is illustrated in FIGS. 6–8,inclusive. In this embodiment, two tube rows are formed and they areconnected in hydraulic series. Again, like components will not beredescribed in the interest of brevity and will be given the samereference numerals as those used previously. In the embodiment of FIGS.6–8, a heat exchanger much like that illustrated in FIGS. 1–3 is formedusing the fins 26 that are provided with slots 34 opening to only oneedge 36 of the fins 26. In this embodiment, tubes 56 extend between theheaders 20, 22. However, the tubes 56 are considerably longer than thoseillustrated in the embodiment of FIGS. 1–3 for a heat exchanger havingthe same frontal area and two stacks of the fins 26 are used. Each stackis abutted against a corresponding one of the headers 20, 22 leaving agap, generally designated 58, in the center of the heat exchanger whichis characterized by the absence of the fins.

Prior to assembling and brazing, the heat exchanger using the methodmentioned previously, the center part 60 of each gap 58 is rotated up toand including 90° and relative to that part of each tube 56 and mergingfrom each of the two stacks of fins 26 to form a bent section 62 closelyadjacent to each of the stacks of the fins 26. The central section 60 ofeach gap 58 is free of a twist as illustrated in FIG. 7. The componentsare assembled and brazed, following which the two headers 20, 22 may bebrought into contact with one another as illustrated in FIG. 7 to form a180° arcuate section 64 between the two twists 62. Thus, the tubes 56are U-shaped with legs 66 being straight and extending between thetwists in a corresponding one of the headers 20, 22 and with the bightof the U being defined by the central section 60 of the gap 58 anddefining the arcuate section 64.

While the embodiment shown in FIGS. 6–8 employs only two rows of thetubes, it will be appreciated that any desired number of rows of thetubes could be provided in the same fashion simply by increasing thenumber of gaps and providing twists 62 and bends 64 in each of the gaps58. For example, a three row construction made according to theembodiment shown in FIGS. 6–8 would have three stacks of the fins 26separated by two of the gaps 58.

FIG. 9 illustrates still another embodiment of the invention. In thiscase, a single tube 70 is formed in serpentine fashion to have aplurality of straight runs, there being eight such runs illustrated inFIG. 9. Headers 20, 22 are located at the ends of the single tube 70 andthe straight runs 72 fitted with fins such as the fins 26. Of course, ifa two row heat exchanger according to FIG. 9 were intended, fins 50employed in the embodiment of FIGS. 4 and 5 could be employed along withan additional one of the tubes 70.

FIG. 10 illustrates a highly preferred form of the fins 26, 50 utilizedin the invention. The fins 26, 50 in this embodiment are arcuate asillustrated in FIG. 10 and include conventional louvers 80 along withspacing legs 82. The fin slots 34 (not shown) in FIG. 10 are free offlanges and abut the side walls 32 of the tubes 24, 56. At this point,during the brazing process, the edges 84 will form a good bond with theflat sides 32 of the tubes 24, 56 when the aforementioned process or aconventional brazing process is employed. It is of some interest to notethat the fact that the fins 26, 50 are arcuate, provides a certainspringiness or resilience to cause the edges 84 to be urged against theside walls 32. Moreover, the absence of flanges on the edges 84increases the air side free flow area to contribute to an improved airside heat transfer coefficient.

FIG. 11 shows a somewhat less preferred embodiment of a fin 26, 50 thatmay be used in the invention. In this particular embodiment, extremelysmall flanges 88 border the slots 34 in the fins and abut the flat sides32 of the tubes 24, 56.

Again, with the embodiment of FIG. 11, the fins 26, 50 are arcuate. Theimportance of this feature is that the arcuate fins increase the airside surface without increasing fin height, i.e., the same number of thetubes 24, 56 may be fitted into a given frontal area even while the airside surface area is increased through the use of the curved fins.Consequently, the increase in area improves heat transfer on the airside while nothing is lost on the second fluid side because the samenumber of tubes 24, 56 may be employed. Moreover, the length of thelouvers is also increased, thereby increasing turbulence and heattransfer. It is to be noted that the air side performance of theembodiment of FIG. 10 is slightly greater than that of the embodiment ofFIG. 11 and considerably better than that of tubes having conventionallysized flanges for the reason that such flanges reduce the available airside free flow area through the heat exchanger.

FIG. 12 shows another sort of flattened tube that may be employed in theinvention. Specifically, the tube is a tear-drop shaped tube 90 havingmultiple ports 92. Again, the slots 94 in the fins 26, 50 are such as tosnugly receive the tube 90, i.e., the slots 94 as they have a depthsomewhat less than the major dimension of the tube 90 and area shapedlike the cross-section of the tube 90. In this embodiment, the width ofthe slot 94 can be made the same or again, just slightly smaller, thanthe minor dimension of the tube 90.

If desired, in the embodiment illustrated in FIG. 12, or in the otherembodiments, one or more elongated stiffening ribs 100 extending thelength of each of the fins 26, 50 can be employed. The stiffening rib isillustrated in both FIGS. 12 and 13. In the case of the embodiment ofFIGS. 4 and 5, the stiffening rib would be located in the center of thefin, between the two rows of slots. In addition to the stiffeningfunction, the ribs 100 enhance condensate drainage when the heatexchanger is used as an evaporator.

Though not shown in the drawings, in multiple tube row embodiments suchas shown in FIGS. 4–8, the tubes in one row may be staggered withrespect to the tubes in one or more other rows. Moreover, in some casesit may be desirable to have the tube major dimensions canted at someangle other than 90° with respect to the longitudinal axis of the fins.

Turning now to FIG. 14, the same illustrates test results for variousfin constructions, including the fin constructions illustrated in FIGS.10 and 11. Standard air face velocity is plotted against a) heat fluxfor entering temperature difference in btu's per square foot per degreeFahrenheit and b) against air side pressure drop in inches of water. Itwill be appreciated that the curved fins of the invention comparefavorably with conventional serpentine fins illustrating that thebonding problems incurred in plate fin heat exchangers are solved by theinvention. It will be particularly noted that in the case ofconventional, flat plate fin, heat exchange performance for fins madeaccording to the invention possess a significant advantage. It will alsobe observed that the fin of FIG. 10 shows an advantage over the fin ofFIG. 11 both in terms of heat transfer and in terms of providing alesser air side pressure drop.

The invention provides a heat exchanger that eliminates round tubeswhich provide a high drag, i.e., increase air side pressure drop andeliminates mechanical bonds typically found in such heat exchangers.Furthermore, the invention allows the use of relatively small finheights to avoid a loss of efficiency that occurs with large finheights. While the heat exchanger of the invention is suited for manydifferent applications, it is particularly used with advantage as anevaporator in that the use of vertical plate fins with stiffening ribsand gaps between the tubes provide for excellent drainage ofcondensation that conventionally occurs in evaporators used inrefrigeration or air conditioning systems.

Manufacturing is simplified in that the fins 26 on the one hand and 50on the other may be made with the same die simply by repeating thestamping operation on both sides of a wider fin. Furthermore, the uniqueadvantage provided by allowing the rounded edges 40 of the tubes toextend slightly past the edges 36, 52, 54 of the fins permits brazing ofthe components without the use of brazing fixtures designed to locatethe fins in a common plane.

The use of curved fins increases the air side surface area withoutnecessitating an increase in fin height and provides an additionaladvantage of inherent resilience causing the edges of the slots in thefins to tightly embrace the side walls 32 of the tubes to further assurea good bond during brazing.

1. A method of fabricating a heat exchanger comprising: a) providing aplurality of generally parallel tube runs of a flattened heat exchangetube having a major dimension and a minor dimension; b) providing aplurality of plate fins, each having a plurality of tube slotsapproximately equal to the number of tube runs, each slot opening to anedge of the associated fin and having I) a shape corresponding to thecross-sectioned shape a tube run to be received in the slot, ii) a depthless than the major dimension of the tube run to be received in theslot, and iii) a width approximately equal to or slightly less than theminor dimension of the tube run to be received in the slot; c) fittingthe tube runs snugly into corresponding slots in each of the fins suchthat an edge of each tube run extends a distance out of the slots inwhich it is received; d) locating the assembly resulting from step c) ona supporting surface with said tube run edges in contact with saidsupporting surface and with said plate fins extending above said tuberuns; and e) subjecting said assembly to an elevated temperaturesufficient to braze said fin to said tube runs while said assembly is onsaid supporting surface and in the absence of brazing fixtures holdingsaid fins and said tube runs in assembled relation.
 2. The method ofclaim 1 wherein said tube runs are defined by straight sections of aserpentine tube.
 3. The method of claim 1 wherein said tube runs areeach defined by a straight piece of tubing.
 4. The method of claim 1wherein the cross-section of said tube runs is tear-drop shaped.
 5. Themethod of claim 1 wherein the cross-section of said tube runs isoval-shaped.
 6. The method of claim 1 wherein said fins are curved atlocations between said slots.
 7. The method of claim 1 wherein said finsand said tube runs are formed of aluminum and/or alloys thereof.
 8. Amethod of fabricating an aluminum and/or aluminum alloy heat exchangercomprising the steps of: a) assembling a plurality of plate fins havingopen ended slots to a plurality of tube runs having the samecross-section shape as the slots such that an edge of each tube runextends a short distance out of the slots in which it is received; b)locating the assemblage resulting from step a) on a supporting surfacewith said tube run edges contacting said supporting surface and saidfins above and out of contact with said supporting surface; c) locatingan aluminum braze alloy at the interfaces of said tube runs and saidfins; and d) subjecting the assembly resulting from the preceding stepsto aluminum brazing temperatures in the absence of brazing fixturesholding said tube runs and said fins in assembled relation for a timesufficient to allow said fins to settle under gravitational forces ontosaid tube runs.
 9. The method of claim 8 wherein step c) is performed bycladding one or both of said fins and said tube runs with said aluminumbraze alloy prior to the performance of step a).
 10. The method of claim8 wherein said tube runs are of flattened tubing having a majordimension and a minor dimension and said slots have a depth somewhatless than said major dimension.
 11. The method of claim 8 including thestep of providing the fins with curved sections between said slots. 12.A method of fabrication of a heat exchanger comprising the steps of: a)assembling a plurality of plate fins having open ended slots to aplurality of tube runs having the same cross-section shape as the slotssuch that an edge of each tube run extends a short distance out of theslots in which it is received; b) locating the assemblage resulting fromstep a) on a supporting surface with said tube run edges contacting saidsupporting surface and said fins above and out of contact with saidsupporting surface; and c) subjecting the assembly resulting from thepreceding steps to brazing temperature in the absence of brazingfixtures holding said tube runs and said fins in assembled relation fora time sufficient to allow said fins to settle under gravitationalforces onto said tube runs.